Density Functional Theory and Molecular Dynamics Simulation of Poly(dimethylsiloxane) Melts near Silica Surfaces

Abstract

Classical density functional theory (DFT) is applied to study properties of fully detailed, realistic models of poly(dimethylsiloxane) liquids near silica surfaces and compared to results from molecular dynamics simulations. In solving the DFT equations, the direct correlation functions are obtained from the polymer reference interaction site model (PRISM) theory for the repulsive parts of the interatomic interactions, and the attractions are treated via the random-phase approximation (RPA). Good agreement between density profiles calculated from DFT and from the simulations is obtained with empirical scaling of the direct correlation functions. Separate scaling factors are required for the PRISM and RPA parts of the direct correlation functions. Theoretical predictions of stress profiles, normal pressure, and surface tensions are also in reasonable agreement with simulation results.

General Methodology for Estimating the Stiffness of Polymer Chains from Their Chemical Constitution: A Single Unperturbed Chain Monte Carlo Algorithm

“…We chose to apply a united-atom force field developed by Curro et al − in our simulations. This is a hybrid force field which mixes class I (e.g., Lorentz–Berthelot) with class II mixing rules and 9–6 with 12–6 Lennard-Jones equations to describe nonbonded interactions.…”

Local Dynamics and Primitive Path Analysis for a Model Polymer Melt near a Surface

“…The number of bonding sites in the simulation was calculated assuming that 4% of the silica molecules covering the surface of the wall are active and react with the bonding molecule. [17] The surface density of active sites is therefore 0.2σ −2 . The resulting bond between a wall particle and a coarse grained bead is thus a soft entropic spring of finite length of the order of several chemical units that is modelled by an non-harmonic spring:…”

“…The bead diameter was mapped through the polymer C ∞ ratio [16] to give σ ∼ 8 Å. We apply a mixing rule using values for the PE and silica interaction intensity found in the literature [17] to obtain a Lennard-Jones potential between the polymer and the wall of σ wall = 0.6875σ and ε wall = 0.82ε. The potential is cut off at its minimum 2 1/6 σ wall so that it is purely repulsive, as expected for the interaction between PE and an untreated silica surface.…”

Structure of Poly(dialkylsiloxane) Melts: Comparisons of Wide-Angle X-ray Scattering, Molecular Dynamics Simulations, and Integral Equation Theory

“…The theory makes use of the generalized Ornstein-Zernike equation [27][28][29] of Chandler and Andersen.…”

“…In this work we use only the repulsive part of the potential in the closure relation. The PRISM theory outlined here has been applied previously to polyolefin 28 and PDMS 8,29 melts. For the present application to silicones, each monomer contains 6 (PDES) or 5 (PEMS) independent united atom sites representing O, Si, CH 2 , and CH 3 moieties.…”